Fuel cells that electrochemically combine hydrogen and oxygen are presently being developed and used for production of electric power in stationary and automotive applications. These power sources are assemblies of a stack of individual cell elements to deliver a power requirement at a specified voltage. The heart of a cell is a membrane electrolyte and electrode assembly (MEA) comprising, for example, a solid polymer, proton exchange electrolyte membrane with a porous catalytic anode on one side of the electrolyte membrane and a porous catalytic cathode on the other side of the membrane. In an assembly of many such cells, each pair of MEAs is separated by a current collector sheet, sealing gasket, and a current collector plate, sometimes called a bipolar plate.
The bipolar plate comprises two thin, facing metal sheets that are shaped to define a flow path on the outside of one sheet for delivery of fluid fuel, for example hydrogen gas, to the anode of one MEA and a flow path for oxygen, often air, on the outside of the second sheet to the cathode side of another MEA on the opposite side of the plate. When the sheets are joined, the surfaces facing between them accommodate the flow of a dielectric cooling fluid. The plates are made of a formable metal that provides suitable strength, electrical conductivity and resistance to corrosion. Stainless steel sheets (316L alloy) of about 0.1 mm gage are an example of a suitable material.
The manufacture of such bipolar plates has been complex and expensive. In one procedure, the metal plate pairs are separately stamped to form the respective fuel and oxidant flow passages and carefully assembled for joining such as by brazing to form a hermetic seal between the two plates. It has also been proposed to roll bond unformed metal sheets using a stop-off or anti-bonding material in areas where the flow passages are to be formed. After roll bonding, which significantly deforms the sheets, a pressurized fluid is introduced between them to expand non-bonded surfaces into suitable flow passages on both the facing and non-facing sides of the sheets. This practice is somewhat simpler than stamping, aligning, and brazing the sheets, but the roll bonding tends to significantly thin the sheets and enlarge their area, and allowance must be made to achieve the desired shape of the bipolar plate for fitting into the fuel cell stack.
It is an object of this invention to provide a simpler method, utilizing a version of roll bonding for fabricating bipolar plates. It is a further object of the invention to provide a method of fabricating bipolar plates that utilizes a pattern of soft bonding material for the metal sheets and lower roll bonding force.